Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

443
FXUS63 KLOT 130510
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1210 AM CDT Mon Jul 13 2026

.KEY MESSAGES...

- Hot conditions are expected for much of the upcoming week;
  though somewhat cooler near Lake Michigan.

&&

.UPDATE...
Issued at 815 PM CDT Sun Jul 12 2026

A recent surface analyses revealed the center of a 1020 mb
surface high positioned directly over the Lower Great lakes.
Such a pressure pattern is leading to a gorgeous mid-summer
evening with clear skies and warm temperatures in the upper 70s
to lower 80s. Meanwhile, a strong upper-level ridge is building
over the northern United States with 500mb heights sampled
around 593dm by the 00Z BIS, ABR, and UNR RAOBs.

Over the next 48 hours, the surface pattern will more or less
stay the same with the center of the surface high pressure
system centered over the Lower Great Lakes. Even with
agricultural evapotranspiration nearing climatological peak
rates and mid-level subsidence associated with the surface
high, am skeptical that dew points will surge into the mid 70s
over the next 48 hours given otherwise full sunshine, drying
soils, and blocked low-level moisture trajectories. (Will note
that current dew points across the area in the upper 50s to mid
60s are already running 3 to 6 degrees lower than those
advertised by NAM and GFS-based guidance). As a result, opted to
lower dew points to largely remain in the upper 60s to locally
70 through Tuesday much in line with RAP/HRRR/RRFS guidance.
Given temperature and dew points have an inverse relationship,
did increase temperatures by a few degrees with forecast highs
now in the low to mid 90s tomorrow and mid to upper 90s on
Tuesday. Of course, daily lake breezes should hold locations
within a few miles of the Lake Michigan shore to the low to mid
80s.

Updated products will be sent soon.

Borchardt

&&

.DISCUSSION...
Issued at 1241 PM CDT Sun Jul 12 2026

An already strong mid-upper level ridge of high pressure over the
Rockies will strengthen further as it builds eastward across the
northern Plains into the western Great Lakes/upper Mississippi
Valley through midweek. Most guidance has 500mb heights peaking
around 6000m over the upper Mississippi Valley. To put into
perspective just how impressive that is, in the nearly 80 year
long record of upper air soundings from the Minneapolis area,
the highest 500mb height ever record was 5980m.

This near-record strong upper ridge will set the stage for
several days of hot and likely humid conditions across much of
the region. The core of this heat dome does look to stay just
north of our area with the highest temp anomalies likewise
expected to remain just north of our area. While the most
intense heat should remain to our north, it still looks likely
to get quite hot locally, especially Tuesday and Wednesday when
high temps are likely to top out in the low to mid 90s across
our entire CWA.

There remains considerable uncertainty in the dewpoint forecast,
which will dictate just how high heat indices will get. Low level
trajectories will be pretty light with the origins of our air mass
solidly continental with no contributions from the Gulf. The
expected increase in dewpoints over the next few days will be
almost exclusively driven by evapotranspiration (ET), a process
which isn`t explicitly modeled, but rather handled by a
parameterization and then fed into the physical model. This
certainly opens the door to potential errors, but given we are
entering the climatological peak of ET, the model guidance that
depicts dewpoints increasing into the 70s by Tuesday and Wednesday
seem fairly plausible. Subsidence associated with the very strong
upper ridge should result in a fairly low inversion and likely
prevent deep mixing which could allow the more shallow low level
moisture to mix out. Ultimately, the NAM overall did a good job
with temps and dewpoints with the heat wave a couple of weeks ago,
so opted to lean heavily on that model for temps/dewpoints through
Wednesday. This looks to result in heat indices peaking in the
upper 90s to slightly over 100 degrees both Tuesday and Wednesday
afternoon.

By Thursday, the GFS is more aggressive with weakening of the
upper ridge and for a couple of runs now depicts a back door cold
front resulting in some meaningful break in the heat late in the
week. The ECMWF and GEM maintain a stronger upper ridge and do not
have a backdoor front on Thursday. Given the spread, the NBM
seems to offer up a reasonable compromise in solutions, but the
spread in potential high temps/peak indices for Thursday and even
Friday is fairly large.

Proximity of the surface ridge should result in weak low level
flow which should allow or lake breezes to form each of the next
several days. While southern Lake Michigan water temps are
currently running close to average, they should warm to solidly
above average in the coming days given the light winds, warmth,
and full sun during the day. The resultant lake breezes will
probably only cool temps slightly into the low/mid 80s, with
that more pronounced cooling likely confined to areas very near
the shore. The magnitude of cooling as well as the timing and
extent of inland penetration of the lake breeze will play a
pivotal role in just how hot it will get over the densely
populated urban corridor of Chicago. If the higher dewpoints do
materialize, then it is possible that portions of Cook County
farther inland from the lake could see heat indices peak near or
just over 100F Tuesday and Wednesday with the potential for
some late afternoon/early evening moderation of the heat
depending on the progress and strength of the lake breeze.

Unless dewpoints overperform even the most aggressively moist
models, then we will likely generally remain below headline
criteria for heat. The one exception could be Cook County away
from the lake which could reach its 3 day Extreme Heat Warning
criteria Tuesday through Thursday, assuming that the backdoor
front doesn`t bring more meaningful relief. Given the potential
for dewpoints to mix out, lake cooling to be more prominent, and
the possibility of a backdoor front on Thursday, the chances of
needing a heat headline for Cook County look to only be about
30-40% at this point.

Late this week and into next weekend medium range guidance is in
pretty good agreement in breaking down the upper ridge with more
of a northwest flow pattern setting up locally. This could result
in a more active pattern locally with chances for showers/storms
increasing late in the week into next weekend.

- Izzi

&&

.AVIATION /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 1210 AM CDT Mon Jul 13 2026

VFR and light winds are expected through the forecast period.
Winds overnight will be light and variable if not outright calm.
After sunrise, they are expected to remain below 10 knots but
prevail out of the northwest. A lake breeze is expected to
develop in the afternoon, which may switch wind directions to
the east-northeast for terminals closer to the lake, but there
is uncertainty on the inland extent for the wind direction
change.

For situational awareness: there is a storm complex presently
crossing the Upper Peninsula of Michigan diving southward toward
the rest of Michigan. No precipitation is expected, but there
is a non-zero chance this complex sends outflow westward that
could flip wind directions to the east for a short period later
this morning. Confidence is too low for any mention in the TAF.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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